1. Field of the Invention
The present invention is directed generally to wireless communication systems. More particularly, the present invention is related to the transmission and reception of physical downlink control channels.
2. Description of the Art
A communication system includes a DownLink (DL) that conveys transmission signals from transmission points such as Base Stations (BS or NodeBs) to User Equipments (UEs) and an UpLink (UL) that conveys transmission signals from UEs to reception points such as NodeBs. A UE, also commonly referred to as a terminal or a mobile station, may be fixed or mobile and may be a cellular phone, a personal computer device, etc. A NodeB is generally a fixed station and may also be referred to as an access point or some other equivalent terminology.
DL signals consist of data signals carrying the information content, control signals carrying DL Control Information (DCI), and Reference Signal (RS) which are also known as pilot signals. A NodeB transmits data information or DCI to UEs through respective Physical DL Shared CHannels (PDSCHs) or Physical DL Control CHannels (PDCCHs).
UL signals also consist of data signals, control signals and RS. A UE transmits data information or UL Control Information (UCI) to a NodeB through a respective Physical Uplink Shared CHannel (PUSCH) or a Physical Uplink Control CHannel (PUCCH).
A PDSCH transmission to a UE or a PUSCH transmission from a UE may be in response to dynamic scheduling or to Semi-Persistent Scheduling (SPS). With dynamic scheduling, a NodeB conveys to a UE a DCI format through a respective PDCCH. With SPS, a PDSCH or a PUSCH transmission is configured to a UE by a NodeB through higher layer signaling, such as Radio Resource Control (RRC) signaling, and occurs at predetermined time instances and with predetermined parameters as informed by the higher layer signaling.
A NodeB also transmits one or more of multiple types of RS including a UE-Common RS (CRS), a Channel State Information RS (CSI-RS), and a DeModulation RS (DMRS). The CRS is transmitted over substantially the entire DL system BandWidth (BW) and can be used by all UEs to demodulate data or control signals or to perform measurements. A UE can determine a number of NodeB antenna ports from which a CRS is transmitted through a broadcast channel transmitted from the NodeB. To reduce the overhead associated with the CRS, a NodeB may transmit a CSI-RS with a smaller density in the time and/or frequency domain than the CRS for UEs to perform measurements. A UE can determine the CSI-RS transmission parameters through higher layer signaling from the NodeB. DMRS is transmitted only in the BW of a respective PDSCH and a UE can use the DMRS to demodulate the information in the PDSCH.
FIG. 1 is a diagram illustrating a structure for a DL Transmission Time Interval (TTI) according to the related art.
Referring to FIG. 1, a DL TTI 100 consists of one subframe 110 which includes two slots 120 and a total of NsymbDL symbols for transmitting of data information, DCI, or RS. The first MsymbDL subframe symbols are used to transmit PDCCHs and other control channels (not shown) 130. The remaining NsymbDL−MsymbDL subframe symbols are primarily used to transmit PDSCHs 140. The transmission BW consists of frequency resource units referred to as Resource Blocks (RBs). Each RB consists of NscRB sub-carriers, or Resource Elements (REs), and a UE is allocated MPDSCH RBs for a total MscPDSCH=MPDSCH·NscRB REs for the PDSCH transmission BW. Some REs in some symbols contain CRS 150, CSI-RS or DMRS. A unit of one RB in the frequency domain and one subframe in the time domain is referred to as a Physical Resource Block (PRB).
DCI can serve several purposes. A DCI format in a respective PDCCH may schedule a PDSCH or a PUSCH transmission conveying data information to or from a UE, respectively. Another DCI format in a respective PDCCH may schedule a PDSCH providing System Information (SI) to a group of UEs for network configuration parameters, or a response to a Random Access (RA) by UEs, or paging information, and so on. Another DCI format may provide to a group of UEs Transmission Power Control (TPC) commands for transmissions of respective PUSCHs or PUCCHs.
A DCI format includes Cyclic Redundancy Check (CRC) bits in order for a UE to confirm a correct detection. The DCI format type is identified by a Radio Network Temporary Identifier (RNTI) that scrambles the CRC bits. For a DCI format scheduling a PDSCH or a PUSCH to a single UE, the RNTI is a Cell RNTI (C-RNTI). For a DCI format scheduling a PDSCH conveying SI to a group of UEs, the RNTI is a SI-RNTI. For a DCI format scheduling a PDSCH providing a response to a RA from a group of UEs, the RNTI is a RA-RNTI. For a DCI format scheduling a PDSCH paging a group of UEs, the RNTI is a P-RNTI. For a DCI format providing TPC commands to a group of UEs, the RNTI is a TPC-RNTI. Each RNTI type is configured to a UE through higher layer signaling (and the C-RNTI is unique for each UE).
FIG. 2 is a block diagram illustrating an encoding process for a DCI format according to the related art.
Referring to FIG. 2, in the decoding process 200, the RNTI of the DCI format masks the CRC of the codeword in order to enable a UE to identify the DCI format type. The CRC 220 of the (non-coded) DCI format bits 210 is computed and it is subsequently masked 230 using the eXclusive OR (XOR) operation between CRC and RNTI bits 240. It is XOR(0,0)=0, XOR(0,1)=1, XOR(1,0)=1, XOR(1,1)=0. The masked CRC is then appended to the DCI format bits 250, channel coding is performed 260, for example using a convolutional code, followed by rate matching 270 to the allocated resources, and finally by interleaving and modulation 280, and then transmission of the control signal 290. For example, both the CRC and the RNTI consist of 16 bits.
FIG. 3 is a block diagram illustrating a decoding process for a DCI format according to the related art.
Referring to FIG. 3, in the decoding process 300, a received control signal 310 is demodulated and the resulting bits are de-interleaved 320, the rate matching applied at the NodeB transmitter is restored 330, and data is subsequently decoded 340. After decoding, DCI format bits 360 are obtained after extracting CRC bits 350 which are then de-masked 370 by applying the XOR operation with the RNTI 380. Finally, the UE performs a CRC test 390. If the CRC test passes, the UE considers the DCI format as valid and determines parameters for a PDSCH reception or a PUSCH transmission. If the CRC test does not pass, the UE disregards the presumed DCI format.
A NodeB separately encodes and transmits a DCI format in a respective PDCCH. To avoid a PDCCH transmission to a UE blocking a PDCCH transmission to another UE, the location of each PDCCH transmission in the time-frequency domain of the DL control region is not unique and, as a consequence, a UE needs to perform multiple decoding operations to determine whether there is a PDCCH intended for it. The REs carrying each PDCCH are grouped into Control Channel Elements (CCEs) in the logical domain. For a given number of DCI format bits, the number of CCEs for the respective PDCCH depends on the channel coding rate (Quadrature Phase Shift Keying (QPSK) is assumed as the modulation scheme). A NodeB may use a lower channel coding rate (more CCEs) for PDCCH transmissions to UEs experiencing low DL Signal-to-Interference and Noise Ratio (SINR) than to UEs experiencing a high DL SINR. The CCE aggregation levels can consist, for example, of 1, 2, 4, and 8 CCEs.
FIG. 4 is a diagram illustrating a transmission process of DCI formats in respective PDCCHs according to the related art.
Referring to FIG. 4, in the transmission process 400, the encoded DCI format bits are mapped to PDCCH CCEs in the logical domain. The first 4 CCEs (L=4), CCE1 401, CCE2 402, CCE3 403, and CCE4 404 are used to transmit a PDCCH to UE1. The next 2 CCEs (L=2), CCE5 411 and CCE6 212, are used to transmit a PDCCH to UE2. The next 2 CCEs (L=2), CCE7 421 and CCE8 422, are used to transmit a PDCCH to UE3. Finally, the last CCE (L=1), CCE9 431, is used to transmit a PDCCH to UE4. The DCI format bits may be scrambled 440 by a binary scrambling code and are subsequently modulated 450. Each CCE is further divided into Resource Element Groups (REGs) (i.e., “mini CCEs”). For example, a CCE consisting of 36 REs can be divided into 9 REGs, each consisting of 4 Res. Interleaving 460 is applied among REGs (blocks of 4 QPSK symbols). For example, a block interleaver may be used. The resulting series of QPSK symbols may be shifted by J symbols 470, and finally each QPSK symbol is mapped to an RE 480 in the control region of the DL subframe. Therefore, in addition to the CRS, 491 and 492, and other control channels (e.g., 493), the REs in the PDCCH contain QPSK symbols corresponding to DCI format for UE1 494, UE2 495, UE3 496, and UE4 497.
For the PDCCH decoding process, a UE may determine a search space for candidate PDCCH locations after it restores the CCEs in the logical domain according to a UE-common set of CCEs (Common Search Space or CSS) and according to a UE-dedicated set of CCEs (UE-Dedicated Search Space or UE-DSS). The CSS may consist of the first CCEs in the logical domain. PDCCHs for DCI formats associated with UE-common control information and use SI-RNTI, or P-RNTI, or RA-RNTI, or TPC-RNTI, and so on, to scramble the respective CRCs are always transmitted in the CSS. The UE-DSS consists of CCEs used to transmit PDCCHs for DCI formats associated with UE-specific control information and use respective C-RNTIs to scramble the respective CRCs. The CCEs of a UE-DSS may be determined according to a pseudo-random function having as inputs UE-common parameters, such as a subframe number or a total number of CCEs in a subframe, and UE-specific parameters such as the C-RNTI. For example, for a CCE aggregation level Lϵ{1,2,4,8} CCEs, the CCEs corresponding to PDCCH candidate m are given by:CCEs for PDCCH candidate m=L·{(Yk+m)mod └NCCE,k/L┘}+i  Equation (1)
In Equation 1, NCCE,k is the total number of CCEs in subframe k, i=0, . . . , L−1, m=0, . . . , MC(L)−1, and MC(L) is the number of PDCCH candidates to monitor in the UE-DSS. Exemplary values of MC(L) for Lϵ{1,2,4,8} are, respectively, {6, 6, 2, 2}. For the UE-DSS, Yk=(A·Yk-1)mod D where Y−1=C-RNTI≠0, A=39827 and D=65537. For the CSS, Yk=0.
The DL control region in FIG. 1 is assumed to occupy a maximum of MsymbDL=3 subframe symbols and a PDCCH is transmitted substantially over the entire DL BW. This configuration limits PDCCH capacity of the DL control region and cannot support interference coordination in the frequency domain among PDCCH transmissions from different NodeBs. Expanded PDCCH capacity or PDCCH interference coordination in the frequency domain is needed in several cases. One such case is the use of Remote Radio Heads (RRHs) in a network where a UE may receive DL signals either from a macro-NodeB or from an RRH. If the RRHs and the macro-NodeB share the same cell identity, cell splitting gains do not exist and expanded PDCCH capacity is needed to accommodate PDCCH transmissions from the macro-NodeB and the RRHs. Another case exists regarding heterogeneous networks where DL signals from a pico-NodeB experience strong interference from DL signals from a macro-NodeB, and interference coordination in the frequency domain among the NodeBs is needed.
A direct extension of the legacy DL control region to more than MsymbDL=3 subframe symbols is not possible at least due to the requirement to support legacy UEs which are not aware of nor support such an extension. An alternative is to support DL control signaling in the conventional PDSCH region by using individual PRBs to transmit control channels. A PDCCH transmitted in PRBs of the conventional PDSCH region will be referred to as Enhanced PDCCH (EPDCCH).
FIG. 5 is a diagram illustrating an EPDCCH transmission structure in a DL TTI according to the related art.
Referring to FIG. 5, although EPDCCH transmissions 500 start immediately after the legacy PDCCHs 510 and are over all remaining subframe symbols, they may instead always start at a fixed location, such as the fourth subframe symbol, and extend over a part of the remaining subframe symbols. EPDCCH transmissions occur in four PRBs, 520, 530, 540, and 550 while the remaining PRBs are used for PDSCH transmissions 560, 562, 564, 566, 568.
A UE can be configured by higher layer signaling the PRBs that may convey EPDCCHs. The transmission of an EPDCCH to a UE may be in a single PRB, if a NodeB has accurate CSI for the UE and can perform Frequency Domain Scheduling (FDS) or beam-forming, or it may be in multiple PRBs if accurate CSI is not available at the NodeB or if the EPDCCH is intended for multiple UEs. An EPDCCH transmission over a single PRB (or a few PRBs contiguous in frequency) will be referred to herein as localized or non-interleaved, whereas an EPDCCH transmission over multiple non-contiguous in frequency PRBs will be referred to herein as distributed or interleaved.
The exact EPDCCH search space design is not material to the claimed invention and may or may not follow the same principles as the PDCCH. An EPDCCH consists of respective CCEs referred to as Enhanced CCEs (ECCEs), and a number of EPDCCH candidate locations exist for each possible ECCE aggregation level LE. For example, LEϵ{1,2,4} ECCEs for localized EPDCCHs and LEϵ{1,2,4,8} ECCEs for distributed EPDCCHs. An ECCE may or may not have a same size as a legacy CCE, and an ECCE for a localized EPDCCH may or may not have a same size as an ECCE for a distributed EPDCCH.
Several aspects for the combined PDCCH and EPDCCH operation in FIG. 5 need to be defined in order to provide a functional operation. One aspect is the process for UE scheduling. As a legacy UE cannot receive EPDCCHs, support of PDCCHs needs to be maintained. However, in many cases, for example in heterogeneous networks, a UE may not be able to reliably receive PDCCHs, or PDCCHs may not exist. Duplicating the transmission of a same DCI format in a PDCCH and an EPDCCH will increase the respective overhead and should be avoided. Moreover, for networks where a macro-NodeB and pico-NodeBs share a same cell identity, the capacity of the legacy CSS may not be sufficient to convey TPC commands to all UEs in the coverage area of the macro-NodeB.
FIG. 6 is a diagram illustrating a network supporting with a same cell identity a macro-NodeB and several pico-NodeBs according to the related art.
Referring to FIG. 6, network 600 includes UE 1 610 which communicates with pico-NodeB#1 615. UE 2 620 communicates with pico-NodeB#2 625. UE 3 630 communicates with pico-NodeB#3 635. Finally, UE 4 640 communicates with the macro-NodeB 645. Although UE1, UE2, and UE3 are within the coverage area of the macro-NodeB, capacity issues may exist for relying on PDCCH from the macro-NodeB due to the resource limitation of the legacy DL control region. In particular, although all UEs in the coverage area of the macro-NodeB can receive SI, RA response, or paging from the macro-NodeB, regardless whether a UE is associated with a pico-NodeB or with the macro-NodeB, the macro-NodeB may not be able to transmit TPC commands to all UEs in its coverage area. Due to the limited number of CCEs in the legacy CSS, transmission of multiple PDCCHs to convey TPC commands to UEs communicating with the pico-NodeBs may not be possible. Moreover, a pico-NodeB cannot transmit its own PDCCHs, as they will interfere with the PDCCHs transmitted by the macro-NodeB.
FIG. 7 is a diagram illustrating an interference co-ordination method in a heterogeneous network according to the related art.
Referring to FIG. 7, heterogeneous network 700 includes UE 1 710 which communicates with pico-NodeB#1 715. UE 2 720 communicates with pico-NodeB#2 725. Finally, UE 3 730 communicates with a macro-NodeB 735. As the macro-NodeB transmits with much larger power than a pico-NodeB, a signal reception at a UE communicating with a pico-NodeB and located near the edge of the coverage area of the pico-NodeB will experience significant interference from signals transmitted by the macro-NodeB. To avoid such interference, the macro-NodeB may blank the transmission of some or all of its signals in certain subframes which can then be used by pico-NodeBs to transmit to UEs located near the edge of the respective coverage areas. For example, the macro-NodeB 740 may substantially reduce (and even nullify) the transmission power of some or all of its signals in subframe 1 745 while transmitting signals with their nominal power in other subframes, while a pico-NodeB may transmit signals with their nominal power in all subframes 750. Subframe 1 is referred to as an Almost Blank Subframe (ABS). ABSs are transparent to UEs and are communicated among NodeBs over an X2 interface in order to facilitate Inter-Cell Interference Coordination (ICIC). ABSs and non-ABSs are indicated using a bitmap spanning a number of subframes such as twenty, forty, or seventy subframes, with a binary 0 indicating, for example, a non-ABS and a binary 1 indicating an ABS.
Another aspect is a variation in a number of REs available for EPDCCH transmissions per PRB, for example, depending on a size of the legacy DL control region, defined by the number of MsymbDL subframe symbols in FIG. 1, on the existence of CSI-RS REs, on the number of CRS REs, DMRS REs, and so on. This variation can be addressed either by maintaining a same ECCE size and having a variable number of ECCEs per PRB (and possibly also having some REs that cannot be allocated to an ECCE) or by maintaining a same number of ECCEs per PRB and having a variable ECCE size.
FIG. 8 is a diagram illustrating variations in an average ECCE size per PRB according to the related art.
Referring to FIG. 8, in a first realization of the contents of a PRB 810, the legacy DL control region spans the first three subframe symbols 820 and there is a first number of DMRS REs 830, CSI-RS REs 832, and CRS REs 834. For 4 ECCEs per PRB, the average number of REs per ECCE is 21. In a second realization of the contents of a PRB 850, the legacy DL control region spans the first two subframe symbols 860 and there is a second number of DMRS REs 870 and CRS REs 872 (no CSI-RS REs). For 4 ECCEs per PRB, the average number of REs per ECCE is 27, or about 29% more than in the first realization. Larger variations in the ECCE size may also exist as the size of the DL control region may be even smaller than 2 OFDM symbols and the number of CRS REs may further decrease.
Therefore, a need exists to define a set of subframes where a UE decodes PDCCH and another set of subframes where the UE decodes EPDCCH.
Another need exists to support transmissions of EPDCCHs in a set of PRBs, from one or more sets of PRBs, while allowing the number of the sets PRBs to vary per subframe.
Yet another need exists to support transmissions of EPDCCHs in one or more ECCEs while allowing a number of REs in an ECCE that can be used to transmit an EPDCCH to vary per subframe.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.